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I figured out what is causing it: in the new 3.9 WP, because I use the ro_RO localization, it somehow forces my cforms plugin to have the same language file in it’s folder domain.tld/wp-content/plugins/cforms/js/langs/ro_RO.js, which I do not have by default.
"Failed to load: https://domain.tld/wp-content/plugins/cforms/js/langs/ro_RO.js" tinymce.js:7761

As I could see, the plugin uses simple names en.js / de.js / fr.js. I manually created the ro.js and it still does not make tinymce work; renaming it to ro_RO.js gets the things working (but it might not make the plugin work if it needs it’s own filename standards).

I do not get it why it forces me to create the language file for a plugin. It should first check if it exists and only then should use that file, just like localizaton works with .mo files, the website does not fail if you miss the file, it will only not use any translation ??

Why do you not have the file loaded by default? Did you delete it from the plugin?

It is not loaded by default, the file does not exist in Cforms unless it is manually created. Only since I updated to 3.9 it is required. So something changed because of that.

Yep, got it.

https://core.trac.www.ads-software.com/ticket/27610

Thanks. In my case I might have found a solution, but most of the users will not, it should work out of the box.

Just to clear things up: Actually the ro.js did not exist previous to my manual creation, that was after I discovered this bug (only en, de, fr and hu existed in the plugin). I put it there just to debug.